It is well known that dipole magnets can be used for particle beam steering in charged particle accelerators, beam lines, and storage rings. Dipole magnets have also been used in magneto-hydrodynamic thrusting devices, for applications such as propulsion of sea going vessels or fluid pumping, and for magneto-hydrodynamic power generation. Additional applications for these magnets have included electrical machinery such as motors and generators. These applications have benefited by the use of superconducting magnets whose higher magnetic fields enhance the performance and can substantially reduce the size, and consequently, the cost, of such equipment.
Conventional superconducting dipole magnets used for these applications employ racetrack shaped coils to produce magnetic fields in the range of 3-10 T. Such coils are usually made with a flat Rutherford style superconducting stranded cable or flat ribbon conductor in order to provide a high current density in a small volume. The geometry of such flat-cable coils dictates that they be wound starting from the inside (or smallest radius) turn to the outermost turn to form a saddle-shaped coil half. Two such halves are mounted opposite to each other to form the dipole magnet. This method of making saddle-shaped racetrack coils (which we will refer to as the “conventional” coil) has at least six drawbacks.
First, for accelerator applications, which need a high field strength but very uniform dipole field (one with a very low content of higher-order multipoles), a complex cross-section for the coil is required. Since the field uniformity in such high-field superconducting magnets is almost completely defined by the conductor placement, the conventional design requires an approximation to a cosine θ current distribution in the coil because such a distribution reduces the higher-order multipole fields. The precise conductor positioning to control the field multipole content in such coils is obtained by the inclusion of expensive precision spacers in the coil straight sections plus complex and expensive special pieces at the ends of the racetrack shaped coils. Fields in excess of about 4 T require more than one coil layer, and thus, the complexity of the design is carried into the successive layers.
Second, the conventional coils are almost exclusively wound with flat Rutherford-type cable. This is done in order to get as many turns as possible of high current carrying capacity conductor around the coil aperture and thus maximize the field that can be obtained with a given amount of superconducting material. However, such cable has the drawback of being relatively expensive to produce and suffers current-carrying degradation of the superconductor as the result of mechanical deformation to produce the flattened cable.
Third, the conventional design requires tight bends for the ends of the inner turns of the coil. When such magnets are made from Rutherford-type cable (flat cable), they require complex coil ends because the large aspect ratio of the cable (a width to thickness ratio that is often of the order of 10) makes it difficult to bend the cable around the ends of the coil. Typically, the flat side of the cable is positioned to follow a computer-generated three-dimensional space curve, often called a “constant perimeter bend”, in order to minimize the distortion in the cable. Precise coil end parts are fitted between the coil turns in the end in order to maintain this geometry. The small bend radius and unavoidable conductor distortion in this design makes it difficult to use A 15 superconductors or HTS materials that are needed for fields greater than ˜10 T because these materials are brittle and their current-carrying capacity is strain dependent.
The fourth drawback is that these conventional coils are difficult and expensive to manufacture and require dedicated expensive tooling to make each of the layers that comprise the dipole magnet. Special winding mandrels are needed for each size coil and “curing presses” are needed to complete the coil construction.
The fifth drawback is that the conventional coils require the application of a high azimuthal pre-stress to the coil in order to eliminate the tendency of the coil to pull away from the pole when the magnet is energized and thus cause a premature quench or training of the magnet. The application and monitoring of such pre-stress is complicated and expensive. The level of the pre-stress employed depends on the intensity of the field and such pre-stress becomes excessively high as the design field approaches 10 T. Thus, the conventional racetrack-shaped coils are not generally used for such high field applications.
The sixth drawback is that the saddle-shaped coils used in conventional magnet designs suffer from an effect called “field enhancement”, where the maximum field seen by the superconductor in the coil is greater than the central field in the dipole. This peak field usually occurs at the pole turn of the innermost coil and is often enhanced further by the tight bend radius of the inner turn as it goes around the coil end. This enhancement is typically of the order of 5%. The result is that the performance of the superconductor is degraded, and more superconductor is required to achieve the same performance that would be attained if there were no peak field effect.
The first drawback applies primarily to accelerator magnets that require a high quality (very uniform) field. The other disadvantages (other five drawbacks) of the conventional racetrack designs (such as the need for high azimuthal pre-stress, tight bend radii, field enhancement degradation of the superconductor performance, and complex manufacturing with dedicated tooling) apply to all applications of such magnets.
Additional requirements for higher field magnets (10 T and above) are indicated as follows: When applied to magnets that employ high field superconducting materials that are brittle in nature (such as Nb3Sn, other A-15 compounds, and HTS materials), the level of pre-stress required could damage the conductor. Furthermore, the high forces necessary to compress the coils azimuthally are difficult to apply and retain, and require expensive dedicated equipment. As a consequence, most magnets designed for accelerator use at such high fields employ a block or flat (not saddle shaped) racetrack coil design also using the flat, Rutherford style cable. However this approach suffers from at least three drawbacks.
First, the coil shape attainable in the block coil type design does not produce a very precise approximation to the ideal current distribution necessary for a very low multipole content field. Thus, field trimming methods are needed to achieve the required field uniformity for accelerator applications.
Second, block coil or flat pancake coils require dedicated tooling to wind and form each coil. In the block coil design for a single aperture magnet, the ends of each racetrack coil have to be bent out of the plane of the magnet aperture to allow room for the particle beam tube. The flat coil design spanning two apertures eliminates this drawback; however, it is useful only for twin aperture magnets with one beam aperture above the other.
Third, the minimum bend radius is limited by the allowable bending strain that can be imposed on the brittle conductor. In the block or flat coil design, the innermost turns necessarily have a small bending radius. This may require that the flat racetrack shaped coils made with A-15 or other brittle materials be wound with un-reacted cable, reacted in place, and then vacuum impregnated with epoxy. Such a procedure is complicated and expensive.
U.S. Pat. No. 5,374,913 to Pissantezky describes superconducting dipole magnets for particle accelerators, having a twin bore flux pipe dipole magnet. This patent provides an informative description, “Background of the Invention” of the state of the art of the development of superconducting accelerator magnets using the racetrack-shaped coil referred to as the cosine-θ coil design, and also discusses the drawbacks of the conventional racetrack coil. Pissantezky's magnet is composed of coils in the form of pipes, one inside the other, in which the dipole field is generated in the space between the pipes. Two magnet bores are inserted in this space such that the fields in each are in opposite directions so as to form a twin bore magnet that could be used in a dual ring accelerator. This coil design does not embody the principles or method of generation of the magnetic field used by the tilted double-helix magnet described herein.
U.S. Pat. No. 6,002,316 to McIntryre describes a complex type of stress management for a superconducting coil in a superconducting accelerator magnet, and indicates the problems associated with the coil structural integrity in high field dipoles. This patent describes a technique to counteract the Lorentz forces in a block coil magnet design. It should be noted here that the double-helix coil design of the subject invention provides a relatively simple method of stress management that can allow it to go to high fields without significant deflection in the coil due to the Lorentz forces.
The subject invention double-helix design is an improvement that can be applied to magneto-hydrodynamic devices for applications such as ship propulsion, fluid pumps, and power generators. Such devices depend on a strong dipole field for operation.
In the current state of the art, conventional racetrack coils are used to generate the magnetic field. For example, U.S. Pat. No. 4,301,384 to Gaines describes a method of the support of the end turns in a superconducting dipole magnet used for magneto-hydrodynamic generation of electrical power, and uses elongated, saddle-shaped superconducting magnet rings that comprise a conventional racetrack dipole coil design. U.S. Pat. No. 5,284,106 to Meng describes a superconducting magneto-hydrodynamic seawater pump to launch torpedoes, and shows racetrack coils as the method of producing the dipole field required to activate the device.
The use of superconducting windings in electrical machinery (such as induction motors and generators) can provide a high output device in a small size compared to conventional machines. Superconducting windings can generate magnetic fields typically 2-5 times stronger than those which can be obtained with normal resistive windings. Since the energy density rises with the square of the magnetic field, such devices can be considered to be 4-25 times more powerful per unit volume of field than normal types of machines. U.S. Pat. No. 5,672,921 to Herd describes the use of epoxy impregnated superconducting coils of a racetrack shape in the rotating armature for a rotary generator. U.S. Pat. No. 5,777,420 to Gambel shows a superconducting induction motor rotor composed of racetrack shaped coils wound with a high temperature superconducting (HTS) material in tape form.
None of the patent references provide solutions to all the drawbacks of conventional prior art systems described above.